Compact discs are known optical memory devices which are widely used particularly with playback and computer devices for retrieving musical and software compositions. Such devices typically comprise only one information carrying layer and, therefore, suffer from a limited amount of recorded data.
Three-dimensional optical memory devices have been developed typically comprising a plurality of parallel, spaced-apart layers having information carrying regions in which optical properties differ from those of adjacent regions of the layers. One example of such a device is disclosed in U.S. Pat. No. 4,090,031. The device comprises a substrate and a plurality of data layers provided on one side of the substrate. Each of the layers comprises data tracks formed of lines of data spots. The data spots are, in turn, formed of either binary coded digital information or frequency or pulse length modulated analog information which is photographically recorded. The data spots are light reflective. Selection of one data track for playback is accomplished by changing the focus of a reading light beam from one data layer to another, or by making the data tracks of materials having different optical properties, the reading light being of different wavelengths for different layers. To this end, different color dyes or different photoluminescent materials are used and corresponding color filters are selectively positioned in front of light detectors.
The device is manufactured in the following manner. The data layers are made of photosensitive, reflective material such as photographic film or another suitable recording material including printing ink. The data spots are formed of light reflecting metal material having a reflecting index different from that of the data layers. The data spots are fabricated by either vapor deposition through a mask having an aperture corresponding to the data spots, or etching through a photoresist mask.
It is thus evident that such a device suffers from multiple over-reflection when retrieving the information from the layers. Indeed, a reading beam would pass through all the layers, wherein each layer is reflective. This results in the number of layers being limited to two or three layers only. Additionally, such a technology based on the use of a photomask does not provide high information density, and is, therefore, not effective. It is appreciated that a process of manufacturing of such a device is very complicated and time-consuming. It is also understood that an extremely expensive technique is required for the production of each layer and, thereby, the entire process is very expensive. Therefore, such process is not suitable for mass production.